In the past decade the density per unit area of electronic devices, such as very large scale integrated circuits (VLSI's), has greatly increased. By some estimates this increase in density has been on the order of 10,000 times what it was earlier. The space or area available outside of a VLSI in which to make the large number of necessary connections to and from it is becoming almost vanishingly small measured by previous standards. Contrary to the density increase of VLSI's, the density of the passive circuit interconnections, such as connectors, has increased (i.e., the parts decreased in size) by only a relatively small factor, for example, less than about 4 to 1. This presents the difficult problem of providing connections to and from the VLSI's which are small enough to fit the spaces available and which are also sufficiently reliable and manufacturable to be economically useful.
As interconnections are made smaller and smaller, the problems associated with manufacturing and assembling these miniature parts seem to grow exponentially. For one thing, a conventional pin and socket connector part, such as a 25 square metal wire-wrap post, has sufficient size and strength to permit it to be made and handled easily with conventional techniques. Typically parts of such "large" size are assembled into connector systems having "large" centers, such as one-tenth by one-tenth inch. But connectors this large and unwieldy are like the "dinosaurs" of a past age in the environment of the VLSI's of today.
The contact resistance of mating parts in an electrical connector is extensively discussed in the literature. See for example the book by Ragnar Holm entitled "Electric Contacts", published by Springer-Verlag. It is highly desirable that the contact resistance remain stable at a very low value (e.g., a few milliohms) throughout the service life of the connector. An important factor in stability of the contact resistance is the character or quality of the interface or mating surfaces of the contacts. These surfaces should be free of contaminants, substantially immune to oxidation or corrosion, and should be held together with sufficient minimum force to insure intimate metal-to-metal contact. These considerations, especially where high quality electronic connectors are involved, lead to the use of gold (or a similar noble metal) in the contact areas and to contact designs which provide normal contact forces for each pair of contacts in the range from about 100 gms to 150 gms (about 4 oz.). The mating forces of the halves of the connector can easily reach a hundred or more pounds where hundreds of pairs of contacts of a single connector are involved. Thus it is highly desirable for a high density connector to minimize the mating or insertion force while maintaining normal contact forces, e.g., about 100 grams.
Where the individual contacts of a connector are made smaller and smaller to achieve higher density, it is more and more difficult to achieve a "safe" minimum contact force. The miniature parts (e.g., pins and sockets) do not have as much mechanical strength as do larger parts. And "strength" usually decreases exponentially rather than linearly as size decreases. Thus all of these factors of size, strength, contact force, uniformity, and stability must be dealt with effectively in designing a high density electronic connector where reliable performance is essential.
One attempt to provide ultra-miniature contacts between mating portions of electrical circuitry is shown in U.S. Pat. No. 3,971,610, to Buchoff et al. in which small individual contact buttons are affixed at selected points to the circuitry to provide mating connections to other parts of the circuitry. These contact buttons are molded of a semi-conductive elastomer, such as silicone rubber, with an admixture of conductive carbon or metal powder. However the volume resistivity of this elastomeric mixture, while low compared to the elastomer material (rubber) alone, is of the order of a million times greater than the volume resistivity of pure copper. Moreover, the elastomer mixture of these contact buttons is far more subject to the ageing effects of elevated temperatures, to physical deterioration over time (oxidation, etc.), and to large, erratic changes in contact resistance, compared to spring contacts of metal. As a result, this elastomeric contact system has had only limited use and is not suitable in applications requiring high performance and where low, stable contact resistance is essential.
It is desirable to provide a high density electronic connector system which provides very low, stable contact resistance together with thermal and mechanical stability, as well as low mating force. It is also desirable to provide an economical and effective method of manufacturing and assembling such a connector system with the precision and uniformity required.